1. Field of the Invention
The present invention relates to an RFID tag substrate that is disposed to a container of drinks or foods to perform wireless communication between a reader/writer, and more particularly to an RFID tag substrate for a metal component that can maintain good communication characteristics of an RFID tag irrespective of a material or presence/absence of contents of a container and is suitable for an RFID tag adopting an electric wave mode, and to an RFID tag including this tag substrate.
2. Description of the Related Art
In general, a resin container formed of, e.g., a PET resin or a metal container, e.g., an aluminum can or a steel can is extensively used as a container for a drink, e.g., a carbonated drink like beer, cola, or soda, a fruit juice, or various kinds of teas, a container of a canned food, or a container of each of various kinds of liquid products.
Moreover, a pouch container formed of a packaging material obtained by laminating a metal layer, e.g., an Al foil on a soft wrapping material, e.g., a resin film has a light weight, is superior in durability and gas barrier properties, and others, can be readily processed, and manufactured at a low cost. Therefore, this container is widely used as a container of not only a food or a drink but also a container of mainly a liquid product, e.g., a detergent or a cosmetic.
Additionally, predetermined commodity information, e.g., a commodity name, components of contents, a producer, a location of manufacture, an expiration date is shown in the form of characters or a barcode on various kinds of containers formed of a resin or a metal. As to indication of this type of commodity information, usually, the information is printed on a container or a packaging body that packages the container, or printed on a label or the like and attached to a container.
However, commodity information or the like is usually indicated in a small size to avoid spoiling, e.g., a design of a container. As a result, an indication area, a size of a character, or the number of characters is limited, and a problem that sufficient information cannot be given arises.
Further, in case of indication using a barcode, the barcode itself must be flatly provided on a container surface to enable reading by a reader. When the barcode has a scratch or a stain, it cannot be read. Furthermore, since an amount of information that can be encoded in the form of a barcode is limited, there is a fixed limit as means for indicating and recognizing commodity information like the example of indication using characters.
As means for solving disadvantages or inconveniences in conventional indication of commodity information and simply and accurately indicating necessary sufficient commodity information, an RFID tag is recently utilized.
The RFID (Radio Frequency Identification) tag is also called an RF tag, a non-contact IC tag, or an IC tag. This tag recognizes or indicates information recorded in an IC chip when a very small communication terminal formed into a tag shape by encapsulating the IC chip and a wireless antenna by using a resin or glass records predetermined information in the IC chip, the tag is disposed to an object, and a reader (a reader/writer) side picks up the recorded information through wireless communication.
In the RFID tag, data of several-hundred bits to several-kilo bits can be recorded in a memory of the IC chip. Since sufficient information or the like can be recorded and the tag does not contact with the reader side, there is no concern about wear, scratches, or damages of a contact point. Moreover, the tag itself does not have a power supply, and hence it can be processed or reduced in size/thickness in accordance with an object.
When such an RFID tag is used, various kinds of information concerning a commodity, e.g., a name of a commodity, a weight, an amount of contents, a name of a producer/seller, a location of manufacture, a date of manufacture, or a beyond-use data/expiration data can be recorded. A wide variety of commodity indication that cannot be given in the form of conventional commodity information using characters or a barcode can be utilized by just disposing the tag reduced in size/thickness to a commodity.
It is to be noted that, as the RFID tag, there are an active type that has a built-in power supply and a passive type that does not have a built-in power supply. Additionally, the tags can be classified into an electromagnetic induction type that uses a frequency band of 135 kHz or 13.56 MHz or a microwave type that uses a UHF band or a frequency band of, e.g., 2.45 GHz depending on a communication frequency to be utilized.
Meanwhile, when such an RFID tag is disposed to a resin container like a PET bottle, it is readily affected by contents, e.g., water in the container. Further, when the RFID tag is disposed to a metal container, e.g., an aluminum can, a steel can, or a pouch container, it is affected by electroconductive properties of the metal container, and hence the RFID tag has a problem that a communication distance varies or accurate wireless communication is disabled.
Specifically, when a metal is present immediately behind the RFID tag, a signal transmitted from a reader/writer cannot recognize an antenna of the RFID tag, performance of the antenna is considerably deteriorated, and an energy of electric waves cannot be received by the antenna of the RFID tag.
Furthermore, electric waves have properties of intensively giving an energy to an absorbent material or substance if such a material is present nearby. Therefore, when water that is a material having a high electric wave absorbency is present immediately behind the RFID tag, water absorbs almost all energy of electric waves.
Therefore, when the RFID tag is disposed to a metal container or it is attached to a PET bottle having, e.g., drinking water filled therein, performance of the RFID tag is deteriorated, thus disabling accurate wireless communication in some cases.
In a microwave type RFID tag that uses a UHF band or a high-frequency band of, e.g., 2.45 GHz in particular, a communication distance is prolonged, whereas communication characteristics are considerably deteriorated due to absorption by water or an influence of a metal as compared with an electromagnetic induction type RFID tag that uses a band of 135 kHz or 13.56 MHz.
Moreover, communication characteristics of the RFID tag are determined by a gain based on an antenna size. Therefore, when acquiring a long communication distance is tried, an antenna size is increased. As a result, a size of the entire tag is increased, thereby making it difficult to reduce a size of the tag.
Here, as a method of avoiding an influence of water or a metal on such an RFID tag, interposing, e.g., a spacer between the RFID tag and a container to separate the RFID tag from water or a metal by a fixed distance can be considered.
For example, in case of a microwave type RFID tag, a nature that electric waves are reflected on a metal container is utilized to separate the IC chip from a container outer surface by a distance corresponding to a ¼ wavelength of a communication frequency, thus decreasing deterioration in antenna performance caused by a metal. Specifically, when the RFID tag that uses a frequency band of 2.45 GHz is approximately 30 mm separated from the container outer surface, deterioration in antenna performance caused by the metal container can be avoided. Therefore, in this case, when a tag substrate on which the IC chip and the antenna are mounted is formed with a thickness of 30 mm, the RFID tag that enables communication without being affected by the metal can be constituted.
Moreover, as the RFID tag that is disposed to a metal container, e.g., an aluminum can or a steel can, an RFID tag dedicated to a metal has been so far proposed (see Patent Documents 1-3). This tag has a configuration dedicated to a metal container including an electric wave shield, thus avoiding an influence of the metal container.
When the RFID tag is disposed to a container, a magnetic flux produced by the RFID tag is generated in a direction piercing the container. Therefore, when the tag is to be disposed to a metal container, for example, a thermal loss that an electromagnetic wave produced by an antenna portion is absorbed into the metal container side occurs, thus deteriorating communication characteristics of the tag.
For example, when an RFID tag 1000 is disposed to a metal container 1001 as shown in FIG. 32(a), each eddy current is induced on a surface of the metal container 1001 due to a magnetic flux generated by the RFID tag as depicted in FIG. 32(b), and this eddy current cancels out the magnetic flux of the RFID tag 1000, and hence a thermal loss occurs.
Thus, as shown in FIG. 33, in a conventionally proposed RFID tag dedicated to a metal container, a magnetic body (a high-permeability body) 2000 or a dielectric material formed into, e.g., a sheet-like shape is arranged on a side of the RFID tag 1000 facing the metal container 1001. As a result, a magnetic flux generated by the RFID tag 100 is transmitted through the magnetic body 2000, thereby avoiding production of each eddy current on the metal container 1001 side.
Additionally, as an electric wave absorber for a built-in antenna in a wireless LAN or a non-contact IC card is also proposed. According to this electric wave absorber, an electroconductive ultra-micronized powder covered with an insulative film is added/mixed in a resin material forming an electric wave absorber to increase a relative dielectric constant of the electric wave absorber, thereby achieving a reduction in thickness of the electric wave absorber (see Patent Document 4).
According to this proposal, when a coating material formed of an electroconductive ultra-micronized powder covered with an insulative film is added/mixed in a resin binder, a dielectric constant of a resin material can be increased while maintaining formability/processability of the resin material. As a result, an influence of a high-frequency noise can be reduced, and the electric wave absorber for a built-in antenna in a wireless LAN or a non-contact IC card can be reduced in size/thickness.    Patent Document 1: Japanese Patent Application Laid-open No. 2002-207980 (pp. 2 to 4, FIG. 1)    Patent Document 2: Japanese Patent Application Laid-open No. 2004-127057 (pp. 3 to 4, FIG. 1)    Patent Document 3: Japanese Patent Application Laid-open No. 2004-164055 (pp. 4 to 5, FIG. 1)    Patent Document 4: Japanese Patent Application Laid-open No. 2005-097074 (pp. 3 to 6)
However, a conventionally proposed configuration of the RFID tag that avoids an influence of moisture or a metal has various kinds of problems.
First, the method of using, e.g., a spacer to separate the RFID tag from a container by a predetermined distance can reduce an influence of moisture or a metal. However, a thickness of a substrate on which the tag is mounted is increased (e.g., approximately 30 mm in case of an RFID tag using 2.45 GHz). When the tag is disposed to a container, it greatly protrudes from the container, and adoption as an actual tag configuration is difficult.
On the other hand, the RFID tag dedicated to a metal container proposed by each of Patent Documents 1 to 3 suppresses occurrence of an eddy current by a magnetic body to enable a reduction in an influence of a metal on an electromagnetic induction type RFID tag. However, this tag cannot cope with an influence of contents (water) in a resin material or an influence of, e.g., reflection of electric waves by a metal container on a microwave type RFID tag.
Further, in such a conventional RFID tag dedicated to a metal, the tag itself is designed/configured for a metal only, and an existing RFID tag cannot be used for a metal container.
That is, a problem that occurs when a regular general-purpose tag is used for a metal container cannot be solved.
Furthermore, the RFID tag dedicated to a metal component has a complicated configuration where a magnetic body or a dielectric material is arranged inside, and has a problem that the tag is increased in size and weight and the most important advantage of the RFID tag, i.e., a small size/thickness and a light weight is deteriorated.
That is, the conventionally proposed RFID tag for a metal component has a larger dimension, e.g., a larger thickness than that of a general-purpose RFID tag. When it is attached to a surface of a metal container, a state that the tag is attached is seemingly clear, thus possibly spoiling an appearance of the metal container. Moreover, the tag may come into contact with other commodities or instruments in shipment or display of commodities, and the tag may be thereby damaged. Additionally, the tag can be detached or damaged artificially, and hence a management system may possibly have a failure.
The RFID tag can exploit its characteristics at a maximum. That is, the RFID tag can be used as wireless communicating means having a low cost, a small size, a light weight, and a large memory capacity when a general-purpose tag that is mass-produced at a low cost is used.
Therefore, a tag having a large wall thickness exceeding 30 mm or a tag dedicated to a metal that has a large/complicated configuration and cannot cope with a resin container considerably diminishes merits of the general-purpose tag.
On the other hand, an RFID tag that is seemingly indistinctive can be reduced in size. However, in this case, a necessary antenna length cannot be assured, and a distance (a range) of wireless communication may be limited to a narrow range, or communication characteristics may be deteriorated due to, e.g., an influence of a neighboring metal container.
Additionally, in regard to the electric wave absorber proposed in Patent Document 4 that adds/mixes an eloctroconductive ultra-micronized powder covered with an insulative film to increase a dielectric constant, specific contents, e.g., a coating thickness of a resin mixed material taking a coating material conformation are not disclosed, and how this electric wave absorber can be utilized with respect to the actual RFID tag is not clear.
Further, Patent Document 4 discloses, e.g., the fact that a dielectric constant at 1 MHz is 45.7, but does not describe how the electric wave absorber can cope with a communication frequency in a GHz band that is used in a wireless LAN or a microwave type RFID tag, and does not solve the above-explained problems in the RFID tag.